Known switch control panels are generally of two types, perforate and imperforate. One type has holes through which movable button actuators project. Foreign particles can enter the interior via the through clearance spaces between the holes and the buttons. Such a panel is difficult to clean. The imperforate type utilizes a thin plate that provides an environmental seal with the case. Such a plate is easily cleaned. The thin plate has suitable legends and actuator regions printed or etched on its surface. Digital pressure at the actuator regions causes flexure which, in turn, causes switch members beneath the plate to engage. Since flexure at the point of application of digital pressure produces proportionate flexure at adjacent regions, some means must be provided for discrete mechanical movement to prevent unintended closure of adjacent switches. One method is shown and described in U.S. Pat. No. 4,249,054 issued Feb. 3, 1981 to Sharp Kabushiki Kaisha as assignee.
In the Sharp structure, a broad area of a stainless steel cover plate is uniformly etched to provide a flexible membrane. A rubber support sheet is laminated to the membrane. A spacer sheet of electrical insulation material is interposed between the membrane laminate and a circuit board. The circuit board has pairs of contact areas or segments arrayed about the board surface. The spacer sheet has access openings about the pairs of contact segments. A conductive rubber pad carried by the membrane laminate is located at each spacer sheet opening. The spacer sheet is just slightly thicker than the pads. If one actuator area of the stainless steel plate is engaged, only its companion conductive pad protrudes beyond the plane of the spacer sheet to engage the contact segments of the circuit board. Adjacent pads remain slightly recessed within the plane of the insulator sheet.
A structure of this type, while entirely operational, requires an extremely thin localized switch actuator for flexibility such as about 0.03 millimeters or 0.012 inches. Metal this thin is easily punctured, and is subject to fatigue failure.
In a structure shown and described in U.S. Pat. No. 4,293,754 issued Oct. 6, 1981 to Sharp Kabushiki Kaisha as assignee, discrete motion is accomplished through removal of the material around all but a portion of the pad. This provides discrete motion but has the disadvantage in that openings are formed around the pad that allow liquid and contaminants to enter the switching system.
The primary object of the present invention is to provide a simple system utilizing a relatively thick imperforate cover plate in which the material of the cover plate itself serves as the bridging contact element.